Micromechanical component and corresponding production method

ABSTRACT

A micromechanical component is described which includes a substrate ( 1 ); a monocrystalline layer ( 10 ), which is provided above the substrate ( 1 ) and which has a membrane area ( 10   a ); a cavity ( 50 ) that is provided underneath the membrane area ( 10   a ); and one or more porous areas ( 150; 150′ ), which are provided inside the monocrystalline layer ( 10 ) and which have a doping (n + ; p + ) that is higher than that of the surrounding layer ( 10 ).

BACKGROUND INFORMATION

[0001] The present invention relates to a micromechanical component and a corresponding manufacturing method.

[0002] Membranes are usually manufactured by bulk or surface micromechanics. Bulk micromechanical designs have the disadvantage that they are relatively complex to manufacture and are therefore expensive. Surface micromechanical variants have the disadvantage that it is generally not possible to manufacture monocrystalline membranes.

[0003] Monocrystalline membranes have the advantage that the mechanical properties are more defined than in polycrystalline membranes. Moreover, it is possible to manufacture piezoresistive resistors having a significantly better long-term stability and higher piezoelectric coefficients using monocrystalline membranes than piezoresistive resistors in polycrystalline membranes.

ADVANTAGES OF THE INVENTION

[0004] The micromechanical component according to the present invention having the features of claim 1 and the corresponding manufacturing method according to claim 7 have the advantage that they make it possible to manufacture a cavern having a superimposed monocrystalline membrane using surface micromechanics simply and cost-effectively. The monocrystalline membrane may be used, for example, for pressure sensors.

[0005] The idea on which the present invention is based is that, to manufacture the membrane, n⁺- or p⁺-doped areas are first selectively anodized (porous etching), which is carried out locally by means of a monocrystalline cover layer, e.g., an epitaxial layer. This is followed by a time-controlled switch to selective electropolishing of an n⁺- or p⁺-doped layer buried under the membrane. In this way, a cavity or a cavern is produced under the cover layer. If necessary, the porous n⁺- or p⁺-doped areas in the cover layer are finally sealed to enclose a defined gas pressure in the cavity produced.

[0006] Additional advantages include simple integration into a semiconductor circuit process, consequently making it possible, for example, to integrate a membrane having an evaluation circuit on a chip (e.g., as a pressure sensor). Little fluctuation due to underetchings occurs, i.e., it is possible to implement exactly specifiable dimensions. Moreover, simple sealing of the access openings is possible, if desired.

[0007] Advantageous refinements of and improvements on the particular object of the present invention are found in the dependent claims.

[0008] According to a preferred refinement, one or more sealing layers are provided above the monocrystalline layer to seal the porous areas.

[0009] According to another preferred refinement, the porous areas are sealed by oxidation. This is a particularly effective sealing method.

[0010] According to another preferred refinement, the monocrystalline layer and the porous areas are of the same doping type.

[0011] According to another preferred refinement, the monocrystalline layer and the porous areas are of different doping types.

[0012] According to another preferred refinement, the monocrystalline layer is provided by epitaxy.

[0013] According to another preferred embodiment, the substrate is of a first conduction type and the buried layer is of a second conduction type. In the buried layer, one or more areas of the first conduction type are provided, which have higher doping than the substrate. This makes it possible to concentrate the lines of force during electropolishing and to avoid undesirable residues in the cavity.

DRAWINGS

[0014] Exemplary embodiments of the present invention are depicted in the drawing and explained in greater detail in the following description in which:

[0015]FIGS. 1a-c show a schematic cross-section of the manufacturing process for a micromechanical component according to a first embodiment of the present invention;

[0016]FIGS. 2a, b show a schematic cross-section of the manufacturing process for a micromechanical component according to a second embodiment of the present invention;

[0017]FIG. 3 shows a possible complication in manufacturing the micromechanical component according to the present invention, and

[0018]FIGS. 4a, b show a schematic cross-section of the manufacturing process for a micromechanical component according to a third embodiment of the present invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0019] Identical reference numerals in the figures denote identical components or components having an identical function.

[0020]FIGS. 1a-c show a schematic cross-section of the manufacturing process for a micromechanical component according to a first embodiment of the present invention.

[0021] In FIGS. 1a-c, reference numeral 1 denotes a p-doped silicon substrate; 5 denotes a buried n⁺-doped layer; 10 denotes an n-epitaxial layer; 15 denotes n⁺-doped areas in n-epitaxial layer 10; 10 a denotes a later membrane area and 20 denotes a mask and 30 denotes a sealing layer of, e.g., metal, oxide, nitride, BPSG, etc.

[0022] According to FIG. 1a, buried n⁺-doped layer 5 in p-silicon substrate 1 is produced under n-doped epitaxial layer 10 by standard process steps, e.g., by implantation. In addition, n⁺-doping areas 15 are incorporated in epitaxial layer 10 at selected sites, e.g., at points or in the form of strips or rings, to produce n⁺-doped connections from the surface to buried n⁺-layer 5. As an option, it is possible to deposit and structure a masking layer 20 or a plurality of such masking layers (e.g., of nitride) on epitaxial layer 1.

[0023] According to FIG. 1b, n⁺-doping areas 15 may be converted into porous n⁺-areas or entirely dissolved by electrochemical etching in solutions containing hydrofluoric acid (“anodization”), depending on the anodization conditions (hydrofluoric acid concentration, current density, etc.). The anodization rate is strongly dependent on the doping of the silicon. Low-doped n-silicon (n-epitaxial layer) is barely attacked while n⁺-doped silicon is readily attacked. This selectivity is used to advantage in this embodiment.

[0024] In a first, time-controlled anodization step, n⁺-doped areas 15 in epitaxial layer 1 are etched to more or less complete porosity. The porosity is preferably greater than 50%. A change of the anodization conditions causes buried n⁺-doped layer 5 to be dissolved away. In the transitional area from n⁺-area 5 to p-doped substrate 1, there is a weakly n-doped area which acts as an anodization limit. The form of buried n⁺-doping 5 defines the area dissolved out.

[0025] According to FIG. 1c, porous areas 150 may—if desired—be closed very simply in a subsequent process step after removal of mask 20 because they have a nearly flat surface having very small holes. This is a significant advantage compared to standard micromechanical surface manufacturing methods in which holes usually having diameters greater than one μm must be sealed. The sealing may be performed, for example, by deposition of metal layer 30 or several layers (oxide, nitride, metal BPSG, . . . ) or by oxidation. The process pressure during deposition defines the internal gas pressure in cavity 50.

[0026]FIGS. 2a, b show a schematic cross-section of the manufacturing process for a micromechanical component according to a second embodiment of the present invention. The dopings used for the etched areas have been varied in this case.

[0027] In addition to the reference symbols already introduced in FIGS. 2a, 2 b, 5′ denotes a buried p⁺-doped layer and 15′ denotes p⁺-doped feed-through areas.

[0028] In this example, a p⁺-doping is incorporated in p-substrate 1 for buried layer 5′. n-epitaxial layer 10 is grown epitaxially over it and provided with p⁺-feedthroughs 15′.

[0029] According to FIG. 2b, p⁺-doped area 15′ is selectively anodized to form porous p⁺-doped area 150′. n-epitaxial layer 10 is not attacked in this case and p-substrate 1 is attacked only slightly since the anodization rate of p⁺ is significantly higher than that of p and n.

[0030]FIG. 3 shows a possible complication in manufacturing the micromechanical component according to the present invention.

[0031] When the buried doping layer is dissolved out, there is the danger that a, silicon web 151 will remain at the point at which two etch fronts meet. This web 151 could cause membrane 10 a not to be completely freed, thus adversely affecting its function.

[0032]FIGS. 4a, b show a schematic cross-section of the manufacturing process for a micromechanical component according to a third embodiment of the present invention.

[0033] In the first embodiment, the danger described in connection with FIG. 3 may be confronted by providing a buried p⁺-doping area 5″ in buried n⁺-layer 5 where the etch fronts meet during the subsequent anodization. This p⁺-doping area 5″ causes lines of force S to be guided selectively during the anodization so that no web remains after the silicon is dissolved out.

[0034] Although the present invention was described on the basis of preferred exemplary embodiments, it is not limited to them but instead may be modified in various ways.

[0035] The described and illustrated embodiments are only exemplary of the manufacturing sequence. As an option, additional dopings may be implemented next to the membrane or in the membrane, for example, to manufacture piezoresistors in the membrane and an evaluation circuit next to the membrane for an integrated pressure sensor. The buried n⁺-doped layer and the n⁺-doped feeds through the epitaxial layer may be designed in such a way that the buried layer is dissolved out through lateral n⁺-etch channels, which are connected with the surface of the epitaxial layer at the channel end via the n⁺-feeds. 

What is claimed is:
 1. A micromechanical component comprising: a substrate (1); a monocrystalline layer (10) provided above the substrate (1), the monocrystalline layer having a membrane area (10 a); a cavity (50) provided under the membrane area (10 a); and one or more porous areas (150; 150′) provided inside the monocrystalline layer (10) having higher doping (n⁺; p⁺) than the surrounding layer (10).
 2. The micromechanical component as recited in claim 1, wherein one or more sealing layers (30) is or are provided above the monocrystalline layer (10) to seal the porous areas (150; 150′).
 3. The micromechanical component as recited in claim 1 or 2, wherein the porous areas (150; 150′) are sealed by oxidation.
 4. The micromechanical component as recited in claim 1, 2 or 3, wherein the monocrystalline layer (10) and the porous areas (150) are of the same doping type (n).
 5. The micromechanical component as recited in one of the preceding claims, wherein the monocrystalline layer (10) and the porous areas (150) are of different doping types (n; p).
 6. The micromechanical component as recited in one of the preceding claims, wherein the monocrystalline layer (10) is an epitaxial layer.
 7. A method of manufacturing a micromechanical component comprising the steps: provision of a substrate (1); provision of a buried layer (5; 5′) on the surface of the substrate (1) having higher doping (n⁺; p⁺) than the surrounding substrate (1); provision of a monocrystalline layer (10) above the substrate (1); provision of areas (15; 15′) having higher doping (n⁺; p⁺) inside the monocrystalline layer (10); selective porous etching of the areas (15; 15′) having higher doping (n⁺; p⁺) to produce corresponding porous areas (150; 150′); and formation of a cavity (50) under the monocrystalline layer (10) and formation of a membrane area (10 a) above same by dissolving out the buried layer (5; 5′ ) using the porous areas (150; 150′) with the aid of an electropolishing process.
 8. The method as recited in claim 7, wherein one or more sealing layers (30) is or are provided above the monocrystalline layer (10) to seal the porous areas (150; 150′).
 9. The method as recited in claim 7 or 8, wherein the porous areas (150; 150′) are sealed by oxidation.
 10. The method according to claim 7, 8 or 9, wherein the monocrystalline layer (10) and the porous areas (150) are of the same doping type (n).
 11. The method as recited in one of the preceding claims 7 through 10, wherein the monocrystalline layer (10) and the porous areas (150) are of different doping types (n; p).
 12. The method as recited in one of the preceding claims 7 through 11, wherein the monocrystalline layer (10) is provided by epitaxy.
 13. The method as recited in one of the preceding claims 7 through 12, the substrate (1) being of a first conduction type (p) and the buried layer (5) being of a second conduction type (n), wherein one or more areas (5″) of the first conduction type (p), which have higher doping (p⁺) than the substrate (1), are provided in the buried layer (5). 